Buy x for y offer (2 for 3 for example)

offer.js

const assert = require('assert');

function offer(x, y, itemsInBasket) {
  return Math.ceil((itemsInBasket / x) * y);
}

assert.equal(offer(2, 1, 1), 1);
assert.equal(offer(2, 1, 3), 2);
assert.equal(offer(2, 1, 4), 2);
assert.equal(offer(2, 1, 5), 3);
assert.equal(offer(2, 1, 6), 3);
assert.equal(offer(3, 2, 1), 1);
assert.equal(offer(3, 2, 3), 2);
assert.equal(offer(3, 2, 2), 2);
assert.equal(offer(3, 2, 6), 4);